There are branded Vodafone UK n900s. They have Vodafone 360 preinstalled, a different firmware number from the UK unbranded ones, and there was some question about whether skype had been quietly disabled (You'd need to ask someone who has one about that to be sure). I would also assume they have the voda-logo somewhere on the casing.

My understanding is that the recent firmware upgrade was a few days late for the voda ones, but that, encouragingly, it did arrive. Which is a vaste improvement on previous arrangements where some upgrades have been completely blocked by carriers. Again, you'd need to check with a Voda-owner to be certain.

You can certainly flash with unbranded firmware if you have branded. The jury is currently out on what that would do to your warranty, since you'd be using the tools provided by nokia for doing it. We have yet to have a test case.